Communication of data is an endemic part of modern society. A communication system is deployed, and used, by which to communicate data. In a communication system, the data is communicated between a sending station, at which the data is sourced, and a receiving station, at which the data is terminated. The sending and receiving stations are connected together by way of a communication channel, and the data that is communicated therebetween is communicated upon the communication channel.
If necessary, the data, prior to its communication from the sending station is converted into a form to permit its communication upon the communication channel. When the data is communicated upon the communication channel, the data is delivered to the receiving station. And, once delivered at the receiving station, the informational content of the data is recovered.
Many different types of communication systems have been developed and are regularly utilized to effectuate the communication of data between sending and receiving stations. And, as advancements in communication technologies permit, new types of communication systems, as well as improvements to existing communication systems, continue to be developed and utilized.
A radio communication system is a type of communication system in which the communication channel that interconnects the sending and receiving station is defined upon a radio link. A radio link forms a portion of the electromagnetic spectrum. Through the use of the radio link upon which to define the communication channel, the need otherwise to utilize a fixed-wireline connection is obviated. Radio communication systems, as a result, are less expensive to install due to reduced infrastructure costs. And, a radio communication system is amenable for implementation as a mobile communication system in which communication mobility is permitted.
Many, if not most, radio communication systems are bandwidth-constrained systems. That is to say, the portion of the electromagnetic spectrum allocated to a radio communication system is limited. The bandwidth constraint sometimes limits the communication capacity of the communication system. When the communication capacity of the communication system is constrained in this manner, the communication capacity of the system can be increased only through more efficient utilization of the allocated bandwidth.
Efforts are made, therefore, to more efficiently utilize the bandwidth allocated to the communication system. Through the use of digital communication techniques, significant communication capacity increases are possible. For instance, when data is digitized and formatted into data packets, the data can be communicated through the formation of packet-switched connections formed between the sending and receiving stations. A multiple increase in the communication capacity of the communication system is sometimes possible.
As increasingly data-intensive communication services are required to be effectuated, additional communication techniques have been proposed and, in some instances, implemented, further to increase the communication capacity of a digital communication system.
Recovery of the informational content of the data communicated in a radio communication system is sometimes complicated due to communication conditions on the radio channel upon which the data is communicated. Fading conditions exhibited upon the radio channel distort the values of the data so that the data, when received at the receiving station, differs in values with the values of the data when sent by the sending station. If compensation is not made for the distortion, the informational content of the data cannot properly be recovered.
One communication scheme whose objective is to facilitate effectuation of data-intensive communication services divides the available bandwidth into sub-bands. Data is communicated upon the separate sub-bands, and such data is modulated independently. The sub-bands, conventionally, are selected to be of frequency ranges small enough so that only marginal inter symbol interference (ISI) is exhibited, as the symbol periods are relatively lengthy. Such schemes are generally referred to as being FDM (Frequency Division Multiplexing) schemes. Conventionally, the sub-bands are separated in frequency by guard bands. Filters of large order are used to separate the sub-bands in the frequency domain.
If the guard bands are eliminated, additional spectrum is available for communication purposes. However, data communicated upon the overlapping sub-bands interfere with each other.
A particular type of frequency division multiplexing, referred to as OFDM (Orthogonal Frequency Division Multiplexing), is a technique in which the sub-bands are orthogonally related to one another. That is to say, OFDM is a technique in which frequency domain samples are placed so that sub-bands are orthogonal.
Conventionally, a sending station of an OFDM system utilizes an IFFT (Inverse Fast Fourier Transformer. And, a receiving station of the OFDM system utilizes an FFT (Fast Fourier Transformer). The orthogonality requirement of an OFDM system makes the OFDM communication scheme vulnerable to frequency offset, a problem regularly occurring at the receiving station. Additionally, when an OFDM system is utilized in a mobile environment in which at least one of the sending or receiving stations involved in the communication of the data is moving Doppler shifting aggravates the frequency offset. As relative speeds increase, Doppler shifting correspondingly increases, and the corresponding frequency offsets are, correspondingly, potentially even larger. Frequency offsets between the sending and receiving stations prevent accurate operation of the demodulator, e.g., the IFFT, and recovery of the informational content of the communicated data is ineffectively performed.
Existing OFDM systems assume constant channel conditions during data transmission. That is to say, the sub-bands upon which the data is communicated are presumed to be time-invariant, i.e., free of fading conditions and Doppler shifting. Actual communication conditions are, however, sometimes quite different.
If a manner could be provided by which to utilize an FDM (Frequency Division Multiplexing) communications while not requiring the use of guard bands and also better taking into account the actual communication conditions on the sub-bands upon which the data is communicated, improvements in communications would result.
It is in light of this background information related to FDM communication schemes that the significant improvements of the present invention have evolved.